Hematopoietic Elements of Acute Radiation Sickness

By Amesh A. Adalja, MD, April 1, 2011

The recent earthquake and ensuing tsunami that struck Japan and damaged a major nuclear power plant in Fukushima has placed radiation-related illnesses and the methods to ameliorate them on the minds of many disaster medicine personnel and the general public as well. While low doses of radiation do not merit treatment and high doses are lethal regardless of treatment, intermediate doses can be survivable if proper treatment is initiated. This brief primer on the hematopoietic syndrome of acute radiation sickness highlights some of the clinical issues that arise in treating those with potentially survivable acute exposures to radiation.

Acute Radiation Sickness

Acute illness does not follow relatively low doses of radiation. Instead, such an exposure may increase a person’s lifetime risk of cancer, beginning with a threshold dose of 100 milli-Sieverts. However, doses 1000-fold higher can cause acute illness and death. Acute radiation sickness (ARS) occurs after a threshold of radiation has been absorbed—usually a total body dose of 0.5-1 Sieverts (Sv). With supportive therapy, people who have been exposed to levels as high as 5-6 Sv can survive.1

ARS refers to a spectrum of pathophysiological effects that are induced when high doses of ionizing radiation interact with the human body. ARS consists of 4 syndromes: hematopoietic, gastrointestinal (GI), central nervous system (CNS), and cardiovascular. Of the 4, the hematopoietic syndrome is the only one that may be reversed through medical intervention.

The hematopoietic syndrome occurs at 1-8 Sv, and is characterized by the following clinical signs and symptoms:2

Nausea, vomiting, and diarrhea

Mucositis, skin erythema, and fever

Lymphocytopenia within 48 hours, neutropenia, and thrombocytopenia within 20-30 days

Impaired wound healing, bleeding, and anemia

Triage Tools

A crude form of triage to determine those who should be treated following a significant radiation exposure is the time from exposure to onset of GI symptoms. Following a dose higher than 2 Sv, most people will vomit. If vomiting ensues within 4 hours of exposure, urgent medical evaluation and treatment is likely needed.2

Because of the effects of ionizing radiation on bone marrow, the decline in lymphocyte count following exposure can also be used for triage as the count follows a predictable, dose-dependent decline. Table 1 stratifies the risk and prognosis from radiation exposure based on lymphocyte counts 48 hours after exposure.2

Table 1: Lymphocyte Counts and Prognosis

48 hr.lymphocyte count

Est'dradiation dose

Prognosis

1000-1500 cells/mm3

1-2 Sv

Good

500-1000/mm3

2-4 Sv

Fair

100-500/mm3

4-8 Sv

Poor

<100 mm3

>8 Sv

Death, even with treatment

A more sophisticated but labor-intensive triage tool is the measurement of dicentricism—the presence of 2 centromeres—in chromosomes, which is a manifestation of chromosomal damage. Unlike lymphocyte counts, which are available in all clinical laboratories, analysis of dicentrics requires specially trained personnel and is not widely available.

Treatment of Hematopoietic Effects of Radiation

Several treatment modalities can be utilized to support and protect the ARS patient from the consequences of radiation-induced suppression of bone marrow. Table 2 summarizes available treatments.2 Treatments in development now include myeloid progenitor cells for infusion and other bone marrow stimulants.

More to Learn

Fortunately, no cases of ARS have been reported in Japan to date. However, if any cases do occur, the methods used for triage and treatment will inform our understanding of the pathophysiology and the treatment of ARS.